The wheel axles of bicycles are attached to dropout members on the ends of wheel axle-supporting members of the bicycle (front forks in the case of the front wheels and chain stays in the case of rear wheels). The dropout members have slots that receive the wheel axle in nested relation at the bases of the slots, and the wheel axle is attached to the dropout members by threaded fasteners that clamp it to the dropout members. The threaded fasteners may be ordinary nuts that thread onto threaded end portions of the axle, or they may special nuts that are parts of a fastening assembly known as a "quick release skewer." A quick release skewer has a shaft that passes though a tubular wheel axle, a plain nut threaded onto one end of the shaft, and a cam nut affixed to the other end of the shaft. The cam nut has an operating arm that, when rotated in one direction, loosens a moveable clamping part of the nut and, when rotated in the other direction, tightens the clamping part of the nut.
Like virtually all mechanical devices, the fastening systems for bicycle wheels are not foolproof; if the fastening system is not properly installed, the wheel can detach from the bicycle while it is being ridden, sometimes with unfortunate consequences. In the case of ordinary nuts, they have to be tightened very firmly, which most people recognize and do. Quick release skewers, on the other hand, require the user to understand how they work in order for him or her to correctly install them. The majority of bicyclists whose bicycles are equipped with quick release skewers know how to operate them and have no difficulty in properly and securely installing the wheels. But there are people who do not know how quick release skewers work and do not operate them properly. Those people think that the operating lever is a handle for rotating the shaft and simply tighten (or attempt to tighten) the nut in that way--they do not use the cam nut feature. Those people are very much at risk of having the wheel detach from the bicycle, because hand-tightening the quick release skewer by simply rotating the cam nut and shaft relative to the threaded nut does not sufficiently clamp the wheel to the dropout.
The foregoing problem has not gone unrecognized. Some bicycle manufacturers have provided projecting bosses on the distal parts of the arm portions of the dropouts that form the axle-receiving slot. The bosses are engageable by the perimeters of the axle fasteners, if the axle should unseat from the slots due to looseness of the fasteners, and prevent the wheel from detaching. Accordingly, an improperly installed wheel is retained on the dropouts.
Although the bosses are highly beneficial for bicyclists who are prone, for one reason or another, to not installing their bicycle wheels properly, they are a nuisance to people who want to take advantage of the convenience of a quick release skewer, i.e., the ability to quickly remove a wheel for maintenance by simply loosening the cam nut with the operating lever and to quickly replace the wheel by tightening the cam nut, in each case without the need to turn the threaded nut about the axis of the shaft. When the dropouts have the bosses, the user has to unthread the threaded nut of the skewer to detach the wheel and then retighten the nut to reinstall the wheel. The purpose of the quick release skewer is largely defeated. Accordingly, many experienced bicyclists file or grind the bosses off the dropouts so that they can take advantage of the skewers. And to restore a good appearance, they also have to repaint the dropouts.